1. Field of the Invention
The present invention relates generally to characterizing semiconductor materials. More particularly, the present disclosure involves improving resist resolution by using a dipolar group coupled to an anion of a photo-acid generator of the resist to force anisotropic acid diffusion process during post-exposure bake.
2. Description of Related Art
A chemically amplified (CA) resist functions through the use of a photo-acid generator (PAG) that reacts with light to create a strong acid. Examples of PAGs used in photoresist layers, include triphenylsulfonium perfluorobutylsulfonate (FIG. 1A), bis(p-tert-butylphenyl)iodoium trifluoromethanesulfonate (FIG. 1B), and 5-norbornene-2,3-dicarboximidyl trifluoromethanesulfonate (FIG. 1C). A PAG has two parts: a chromophore, which interacts with the light and creates a strong acid, and an ion, which associates with but does not quench the acid. The strong acid and spectator ion diffuses through the resist polymer and catalyzes approximately 100 chemical reactions before the acid is neutralized, changing the solubility of the resist polymer. In this way, one photon of light creates approximately 100 chemical reactions, thus the term “chemical amplification.” As deep ultraviolet (DUV) 193 nanometers and extreme ultraviolet (EUV) light sources are relatively dim, CA resists have become a necessary component to viable production processes.
In order for multiple reactions to be catalyzed, it is necessary for the acid created in the exposure process to diffuse. This diffusion has been calculated to be about 20-50 nanometers depending on the resist and the process conditions. Thus, CA resists have a resolution limit of about 40 nanometers (half-pitch). The 2004 International Technology Roadmap for Semiconductors (ITRS) calls for 35 nm gates in resist and 45 nm half-pitch lines in 2010. These features are anticipated to shrink 25 nm and 32 nm, respectively three years later. It is currently proposed that 193 nm immersion lithography will handle the 45 nm node in 2010 and EUV will be to production tool of choice to print at the 32 nm node in 2013. Both these systems will use chemically amplified resists in some form.
In general, the acid diffusion needs to be controlled in order to refine the process for the smaller device sizes predicted. Currently, acid diffusion can be controlled by lowering post-exposure bake (PEB) temperatures, use of large PAG molecules, and the addition of a base (acid quencher) to the photoresist. However, these methods slows photo speed, and thus can negate the advantage of the high speed of CA resists (dose-to-size of a line/space pattern <30 mj/cm 2). Moreover, vertical acid diffusion is necessary to smooth out standing waves that are naturally produced by the exposure tool, as shown in FIG. 2A. Current production methods rely solely on a isotropic (non-directional) processes to control acid diffusion such as those mentioned above in this paragraph (FIG. 2B).
Any shortcoming mentioned above is not intended to be exhaustive, but rather is among many that tends to impair the effectiveness of previously known techniques for characterizing substrates; however, shortcomings mentioned here are sufficient to demonstrate that the methodologies appearing in the art have not been satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.